SELECT
    CASE
        WHEN TB.DEPT_CODE IS NOT NULL THEN TB.DEPT_CODE
        WHEN TD.DEPT_CODE IS NOT NULL THEN TD.DEPT_CODE
        WHEN TS.DEPT_CODE IS NOT NULL THEN TS.DEPT_CODE
        ELSE TB.DEPT_CODE
        END AS DEPT_CODE
    , TD.DEPT_NAME AS DEPT_NAME
FROM
    TBL_DEPT AS TD
    RIGHT OUTER JOIN
        TBL_ORDERS AS TB
        ON TD.DEPT_CODE = TB.DEPT_CODE
    RIGHT OUTER JOIN
        TBL_SALES AS TS
        ON TD.DEPT_CODE = TS.DEPT_CODE
GROUP BY
    TD.DEPT_CODE
    , TB.DEPT_CODE
    , TS.DEPT_CODE
ORDER BY
    DEPT_CODE ASC